In an imaging apparatus, such as a digital camera, an image signal output from an image device, such as a CCD (charge coupled device) image sensor, has a low signal level when a subject is dark. Accordingly, an influence of a noise component due to a dark current or random noise included in the image signal increases and thus an S/N ratio (signal to noise ratio) is deteriorated. On the other hand, if the subject is bright and the signal level is high, a dynamic range of the image signal is suppressed by saturation of a light receiving pixel, a CCD shift register or the like.
For this reason, in order to maintain the signal level of the obtained image signal in a preferable range even in a case in which the brightness of the subject is changed, an auto exposure control is performed. This auto exposure control is performed by expanding/contracting an exposure time of the imaging device. Alternatively, the level of the image signal may be controlled by controlling a gain of the image signal in an image signal processing circuit for processing the image signal.
In the latter, the image signal processing circuit includes an analog signal processing circuit which processes an analog image signal output from the imaging device and a digital signal process circuit which is provided at a rear stage thereof and processes the image signal after A/D conversion, and the gain is controlled in the respective circuits. Specifically, in the analog signal processing circuit, an auto gain control (AGC) circuit for amplifying the image signal by a variable gain (analog gain) is provided. On the other hand, in the digital signal processing circuit, a digital gain control (DGC) circuit for multiplying the A/D converted image data by a variable gain (digital gain) is provided.
In the image signal processing circuit, the image data output from the DGC circuit is subjected to an integration operation for one screen, and the auto exposure control circuit controls a driving unit of the imaging device to expand/contract the exposure time based on the integration result, or controls each gain of the AGC circuit and the DGC circuit, and performs a feedback control such that an average level of one screen of the image signal becomes a desired level.
Here, the process of multiplying the image signal by the gain using the AGC circuit or the DGC circuit also amplifies the noise component. On the contrary, the expansion of the exposure time has a small influence on the increase of the noise. For this reason, if the subject is dark, the exposure time is first increased, and, if the level of the image signal is low even though the exposure time becomes an upper limit value, the gains of the AGC circuit and the DGC circuit are increased. On the other hand, in a case of decreasing the level of the image signal, the gains of the AGC circuit and the DGC circuit are decreased, and, if the level of the image signal is too high even though the gains of the AGC circuit and the DGC circuit are set to a lower limit value, the exposure time is shortened.
As described above, when the level of the image signal is increased, the increase of the gains of the AGC circuit and the DGC circuit is disadvantageous in maintaining the S/N ratio. Accordingly, the exposure time is expanded prior to the increase of the gains. However, since the imaging apparatus photographs a moving image for a predetermined frame period according to a vertical scanning period V, the settable exposure time has the upper limit determined according to the vertical scanning period V.
As regard this, there is an idea that the vertical scanning period should be enlarged such that the exposure time is more expanded, if the exposure time reaches the upper limit according to any vertical scanning period V. FIG. 1 is a state transition diagram illustrating a conventional exposure control technology based on the above idea. In FIG. 1, a horizontal axis represents a total gain G of the exposure control and a right direction represents a direction that the gain increases. The total gain corresponds to a product of the exposure gain according to the expansion/contraction of the exposure time and each processing gain of the AGC circuit and the DGC circuit. Also, a plurality of control states for performing the exposure control are arranged deviated in a vertical direction. In order to increase the signal level to a target level, an iris control operation P2 for expanding the exposure time E is performed. If the exposure time E reaches an upper limit value Emax, an enlarged iris control operation P4 for enlarging the vertical scanning period V from a standard value Vst to a value Vex is performed. In the enlarged iris control operation P4, the exposure time E can be expanded from the upper limit value Emax set in the iris control operation P2 to an upper limit value Emax′ according to a new vertical scanning period Vex. In the enlarged iris control operation P4, if the signal level is not enough even though the exposure time E reaches the upper limit value Emax′, the AGC operation P6 is performed and thus the gain thereof is increased from a standard value. In addition, if the signal level is not enough even though the gain of the AGC operation P6 is set to the upper limit value, the DGC operation P8 is performed and thus the gain thereof is increased from a standard value.
In the exposure control, as the subject becomes dark, the total gain of the auto exposure control operation is needed. That is, in FIG. 1, the left side is the side that the subject is bright and the right side is the side that the subject is dark. In the conventional technology, if the subject becomes dark, the enlarged iris control operation P4 is performed before the gain of the AGC circuit or the DGC circuit is varied. That is, in a step that the subject is relatively bright, the vertical scanning period is enlarged and thus a frame rate is reduced. For example, in photographing a still image, a relative large photographing interval is allowed. But, in photographing a moving image or preview, the above-mentioned technology may be disadvantageous, because it is important that the image is smoothly in accordance with the motion of the subject.